In the field of flow and pressure measurement it is well known to employ a hot wire anemometer to determine flow rates. A fluid is typically passed over a single heated wire, reducing the temperature of the wire. The change in resistance of the heated wire is determined and correlated with the flow rate of the gas. A more advanced technique employs two temperature sensing elements located a fixed and equal distance from a heat source. The fluid is passed through the system, reducing the temperature of the upstream sensor and increasing the temperature of the downstream sensor. The temperature difference is then recorded as an output signal.
A major drawback of hot wire anemometers is the nonlinear, temperature and fluid-dependent manner in which they respond to fluid flow. A technique and method for linearizing output signals of such anemometers is disclosed in commonly assigned U.S. patent application Ser. No. 07/611,425 filed on Nov. 11, 1990, now abandoned entitled "Methods and Systems for Fluid Identification and Flow Rate Determination," the disclosure of which is herein incorporated by reference. Unfortunately, this technique does not adequately compensate for drift in the output signal associated with variations in operating temperatures and pressures (while this is typically referred to as ambient temperature and pressure, actual operating temperatures and pressures may vary with instrument construction).
In analytical instrumentation, there is a need for very accurate fluid flow which is compensated for changes in ambient, or current operating, temperature and pressure. In a gas chromatograph, the flow rate of the carrier gas is typically controlled by adjusting the pressure of the carrier gas updstream of a flow sensor. FIG. 1 illustrates a control valve 5 for controlling the flow of fluid 10 from a source 15 (illustrated as a cylinder of pressurized fluid, alternatively, fluid flow could be caused by a negative pressure on the downstream side of the control valve 5). The fluid 10 flows through a mass flow sensor 20 which generates an output voltage 25 corresponding to the mass flow of the fluid 10. The output voltage 25 provides feedback to control the opening and closing of the valve 5. As well known in the control feedback art, the ability of the mass flow sensor 20 to accurately sense and provide the output voltage 25 is very important to controlling flow and pressure.
The repeatability of the chromatographic output of the forward pressure regulated chromatographic apparatus 10 shown in FIG. 2 depends upon the output of the flow sensor 16. The sensor 16 may or may not be resident on the analytical instrument. The computer or microprocessor 24 then generates a feedback control signal 26 for controlling the opening and closing of valve 14 for regulation of the carrier fluid flow. The injection port 12 provides a portion of the carrier fluid/sample combination to a column 18, with the remainder passing through a non-analyzed output 20. Unfortunately, the feedback signal output by the flow sensor drifts with temperature variations and makes it difficult to accurately control the valve 14 and the corresponding flow rate. There exists a need for more stable inlet fluid flows and reduced manifold temperature variations to provide better chromatographic area repeatability as measured by the detector 30 at the end of the column 18.
One method for eliminating temperature sensitivity is to enclose the flow sensing devices in a temperature controlled zone, e.g., a "heated zone" constructed with thermally insulating material. Temperature sensors and heaters inside the heated zone provide feedback to maintain the flow restrictor and pressure sensor temperatures constant and thereby remove temperature as an error-producing variable.
Unfortunately, the incorporation of a heated zone increases manufacturing costs related to instrument calibration and components. Additionally, instrument reliability is reduced as the components required to regulate a heated zone are more likely to fail with continual operation at manifold temperatures higher than ambient. Furthermore, a heated zone requires a long start-up time components required to regulate a heated zone are more likely to fail with continual operation at manifold temperatures higher than ambient. Furthermore, a heated zone requires a long start-up time to stabilize prior to instrument operation.
A need exists for a flow sensor which automatically compensates for ambient temperature and pressure changes without the use of a heated zone.